1. Field of the Invention
The present invention relates to a differential mechanism for a vehicle, particularly to a 4WD vehicle with two auxiliary assist motors.
2. Description of the Related Art
In general, a differential is provided on a four-wheel drive (4WD) vehicle for splitting (dividing or distributing)and transferring a drive force of an engine to front and rear wheels. The differential has an input shaft to which a rotational drive force from the engine is transferred and output shafts to which the rotational drive force of the input shaft is transferred and which are allowed to rotate at different speeds. This provision of the differential allows the front and rear wheels to turn at the different speeds even if there occurs a difference in a turning speed between the front and rear wheels when the vehicle goes round a bend (corner), whereby the vehicle is allowed to run smoothly round the bend.
In recent years, 4WD vehicles are known in which the torque split (distribution) between front and rear wheels is controlled. According to a differential mechanism of the 4WD vehicles of this type, in addition to the differential described above, an oil pump adapted to be driven by the engine is provided to control a plurality of clutches by hydraulic pressure obtained from the oil pump so as to change the rotational driving force transferred to the respective output shafts, whereby a desired torque split can be realized.
With the aforementioned differential mechanism, however, the control of the plurality of the clutches through the hydraulic pressure causes a problem that not only a structure of the differential mechanism but also the torque split control are made complicated. In addition, on top of the complicated structure, there is also caused a problem with securing a sufficient level of reliability due to leakage and deterioration of oil used.